Recently, nonvolatile memory devices have been used for the electronic storage of data. Flash memory is a common example of nonvolatile memory. Since the data contained in the memory device can be of a sensitive or confidential nature, providing security from tampering with or accessing of the data is desired. The semiconductor integrated circuit with a packaged nonvolatile memory chip and logic IC chip disclosed in Japanese Unexamined Patent Publication No. 2000-215108 includes a comparator means for making a comparison between reference data pre-stored in the nonvolatile memory chip and collation data input from outside; and a means for allowing or refusing readout of data stored in the nonvolatile memory chip in accordance with the result of the comparison made by the comparator means. This enables a security function in which data output is allowed when access is attempted by authorized users and refused when access is attempted by unauthorized users.
Another related art is disclosed in Japanese Unexamined Patent Publication No. 211 (1999)-85620.
The above prior art techniques are associated with a security function according to which a password, which is reference data, is pre-stored in the nonvolatile memory chip and compared with collation data input from outside, thereby identifying an authorized user access.
However, the prior art techniques do not teach how the storage reference data (hereinafter referred to as “a password”) in the nonvolatile memory chip can be accomplished. Furthermore, they do not suggest a means of inhibiting fraudulent password acquisition, so that a password can be effectively protected from a risk of acquisition by unauthorized users.
Generally, a password is a string of bit data that is usually stored in a continuous manner in a desired memory cell area of a memory cell array. Therefore, if the address of the memory cell area or the address of a particular memory cell in the memory cell area is fraudulently obtained, the password can easily be discerned from the bit data stored in the area having the address or the area anterior or posterior to the address.
As such, there exists a need for a device and a means of securing a password pre-stored in a nonvolatile memory chip from fraudulent discovery and use.